A simple method for microwave-assisted preparation of tire samples

Heavy metals content in tires affects the safety of soil and agricultural products. The digestion method is a pretreatment for determining heavy metals in tire samples, and will affect the efficiency and accuracy of the heavy metal determination. The microwave digestion process and reagents for tire samples are not currently standardized. Therefore, this study attempts to provide an appropriate method of resolution for scholars. All digestion processes were performed in Mars One. We tested 15 different acid mixtures to determine the best reagent type and dose and then investigated the effect of maximum temperature, holding time, and sample grams on the degree of digestion. In summary, the best condition to digest the tire sample was a mixture of 3 ml HNO3 and 7 ml H2SO4, taking 0.1 (± 0.0005) g tire sample, at the maximum digestion temperature of 220 °C for 25 min. The experimental conclusion will provide a reliable experimental method for scientists using MARS One to study heavy metals in tires. At the same time, researchers using the MARS series can also find valuable references in this paper.


Instrumentation
Tire digestion experiments were conducted in a benchtop microwave digestion system (CEM Mars one, manufactured by CEM Corp., USA) with a maximum power of 1000 W and a temperature control system to detect and control the temperature conditions in the sample container.The turntable supplied by CEM can hold up to 16 digestion vessels.The vessel body and gasket are made of polytetrafluoroethylene (PTFE), and the lid is

Quality control
The reagents and chemicals used were of analytical grade with a purity of 99%.To minimize the risk of contamination, all containers were soaked in 10% HNO 3 (65-68%) for 12 h, rinsed three times with deionized water, and dried in an oven at 60-65 °C for 24 h.The test was repeated three times for each sample.
After grinding and sieving, the sample weighed (0.1 ± 0.0005) grams (g), was accurately weighed, and the sample quality relative error was not more than 0.001 g; that is, the close error of sample quality was not more than 0.5%.In this way, we were able to ignore the effect of the weighing error.
A total of 0.1-0.5 g of solid sample and 5-10 ml of reagent were added, using an even number of vessels for each digestion.To avoid explosion and other hazards, we set the maximum temperature of the instrument below 230 °C.In addition, after each digestion, the digestion tank could only be removed if the temperature was below 80 °C within 20 min.

Experimental method
Generally, the digestion of tires is done by microwave digestion method, but the digestion effect of different types of microwave digestion instrument and different digestion reagents is different.It is widely accepted that a complete digestion is yellowish-white or clear and free of solid residues [49][50][51][52][53] .We determined the best method of microwave digestion of tire by changing the parameter conditions of microwave digestion each time, including reagent type, reagent dose, the digestion procedure (temperature, time), and sample gram number.

Influence on microwave digestion of different acid systems
After grinding and sieving, we placed 0.1 ± 0.0005 g tire samples in the digestion vessels.Then we added different combinations of five acids in the digestion vessels to investigate the influence of different types of acids on the degree of microwave digestion.Following the instrument's instruction manual, we set the initial digestion procedure as shown in Table 1.
The mixture of HNO 3 and H 2 SO 4 had a good digestion effect on the tire samples (Fig. 3).Using a mixture of 6 ml HNO 3 and 1 ml H 2 SO 4 and a mixture of 5 ml HNO 3 and 3 ml H 2 SO 4 changed the colour of the liquid from dark to brown.Furthermore, using the mixture of 5 ml HNO 3 and 3 ml H 2 SO 4 left only a small amount of black residue in the digestion solution.Therefore, we adjusted the dosages of HNO 3 and H 2 SO 4 to explore the best acid system.It is worth noting that when the mixture of HNO 3 and H 2 SO 4 was added to the digestion vessel, an exothermic reaction occurred.Therefore, we needed to place the digestion vessel in a fume cupboard for half an hour to achieve the role of predigestion.
As shown in Figs. 3 and 4, the degree of digestion improved, as the consumption of HNO 3 decreased and the consumption of H 2 SO 4 increased.The mixture of 3 ml HNO 3 and 7 ml H 2 SO 4 had almost completely dissolved the tire, with only a small solid residue, which was light yellow after a constant volume of 25 ml through ultrapure water.Having determined the best combination of acids, we researched the influence of digestion temperature and holding time on digestion to optimize the digestion scheme further.

Influence of temperature and time on the microwave digestion rate
Polymer digestion processes reach high temperatures, and combustion is a very efficient way of destroying matrices (including organic additives) 47,54 .Therefore, the complete dissolution of the tire sample often depends on the highest temperature and holding time during digestion.

Influence on microwave digestion of maximum temperature
Step 1 remains the same, and the changes in Step 2 are shown in Table 2.After grinding and sieving, we added 0.1(± 0.0005) g tire samples and a mixture of acids in digestion vessels.After predigestion, the vessel lid was tightened and placed in the microwave digestion apparatus, and different maximum temperatures were set for digestion.
Too low a temperature affects the degree of digestion, while too high a temperature increases the cooling time and the pressure inside the instrument, increasing the risk.Therefore, we set the maximum temperature to between 180 and 220 °C.
As shown in Fig. 5, when the temperature was below 200 °C, there was still a small amount of solid residue in the digestion solution after digestion, and the tire was not fully digested.As the temperature rose, the effect of digestion improved.When the temperature reached 210 °C and 220 °C, the tire was completely digested, and the liquid had no solid residue.The liquid was colourless and transparent when diluted to 25 ml with ultrapure water.The higher the temperature, the higher the degree of digestion, so we chose 220 °C as the best temperature for tire digestion in a safe and stable experiment.

Influence of holding time on microwave digestion
The holding times in Table 3 were optimized to maximize efficiency under the assumption of complete digestion, as the comprehensive digestion program is mainly influenced by the retention time at the highest digestion temperature.As shown in Table 4, digestion was carried out by setting different holding times.In Fig. 6, under the effect of high temperature, the tire sample was almost completely dissolved even if held for only 10 min.When the holding time reached 25 or 30 min, the liquid appeared virtually colourless and transparent, with the highest degree of digestion.After the digestion solution was diluted to 25 ml, it was still colourless and transparent.Therefore, we chose 25 min as the maximum temperature holding time to achieve the highest resolution in the shortest time.
From the result in Fig. 7, the two factors separate into two groups: (a) higher temperatures result in higher digestibility and clearer liquid and (b) longer holding time, higher digestion level, clear liquid.Therefore, we determined out the best microwave-assisted heating program (Table 5).

Influence on microwave digestion of grams of sample
Practical experience has shown that it is impossible to guarantee that each weighing is exactly 0.1 g, so the microwave digestion program must have some ability to resist the influence of sample mass variations.Moreover, the digestion process is also influenced to some extent by the grams of the tire sample.Therefore, we researched the influence of different grams of samples on digestion.The treated tire samples were weighed in grams and placed in digestion vessels.After adding acid, digestion was carried out according to the microwave-assisted heating program (Table 5).The results are shown in Table 6.In Fig. 8, we found that samples could be wholly digested without residue when the gram was between 0.1 and 0.14 g.After diluting the digestion solution to 25 ml with ultrapure water, the solution was colourless and transparent.The effect of digestion was the best when the gram of sample was 0.1 g.However, when the gram increased to 0.16 g and 0.18 g, the sample could not be completely digested, and a small amount of white solid residue appeared in the digestion solution.

Discussion
In the past, microwave digestion technology was often adopted for the pretreatment of animal, plant, and soil samples 55 .At present, researchers have proposed many microwave digestion schemes for some complex materials, such as spodumene, particulate matter (PM 2.5 ), aquatic products, coke, and so on.The details are shown in the following Table 7.We can see that HNO 3 is the most common acid in sample digestion, and it is a strong   oxidizing agent to release elements in samples as soluble nitrates, and is well motivated by microwave 56 .Concentrated acids (HNO 3 , H 2 SO 4 , HCl and HF), mixed or not, are used in most complex sample digestion methods, increasing the efficiency of sample digestion 57 .Digestion results are closely related to acid type, temperature control, and other operational details 63 , especially for complex samples such as tires.Moraes studied the digestion effect of two acids mixtures based on holding them at 280 °C for 15 min (sample mass was 400 mg) 47 .The reagent volumes were: (i) 5 ml HNO 3 , 1 ml H 2 SO 4 , and (ii) 5 ml HNO 3 , 1 ml HCl and 1 ml H 2 O 2 .There are also a number of scientists involved in rubber tires research, as shown in Table 8.By analyzing all these methods, we concluded that HNO 3 and H 2 SO 4 positively affect the digestion of some complex samples.Furthermore, with almost all temperatures approaching 200 °C or above, the temperature seems to be the biggest factor that affects digestion.Neither of the two acids mixtures in Moraes' study was good at dissolving samples, but they were not investigated further.The sample mass of Nos. 1 and 2 in Table 8 is a range, the maximum temperature of No. 3 is also a range, No. 6 and No. 7 do not even give the grams of the sample, and No. 5 is no acid.In addition, the temperature of some methods is too high, which can pose safety risks.Therefore, we recognized that current rubber tires research is not comprehensive, and the microwave digestion process and reagents for tire samples are not currently standardized.
As a result, this study is a good complement to the research on rubber tires.We combined the study of complex samples and rubber tires and went through 15 acids mixtures to find the best one.At the same time, we refined the two factors of temperature and holding time to find the best solution.Moreover, compared to these methods, the method in this paper not only analyses the selection range of the gram of the sample but also avoids the use of dangerous and environmentally unfriendly HF, H 2 O 2 65,66 .

Conclusions
By changing the conditions of microwave digestion one by one, this study carried out much experimental work and finally determined the best scheme for microwave digestion of tire samples as follows: • Take a 0.1 (± 0.0005) g tire sample.
• A mixture of 3 ml HNO 3 and 7 ml H 2 SO 4 was prepared.
• Control the highest digestion temperature at 220 °C • Hold for 25 min In this way, the tire samples were completely dissolved, the digestion solution was colourless and transparent, and a constant volume of 25 ml was also colourless and transparent.
The microwave digestion program can resist the interference of sample gram fluctuation.It is suitable for the pretreatment process of heavy metal detection of tire samples.The digestion process is characterized by safety, stability, high energy savings, and so on, which is suitable for general popularization.
However, there are some limitations to this study.As cars are a major contributor to traffic emissions, only car tires were selected as the test objects in this research.With the rise of electric vehicles, in the future, we can divide tires into cars, electric vehicles, Goods vehicles, and motorcycles for research purposes.In this paper, only the most representative Michelin tires were selected and only one microwave instrument was used.Thus, comparing different instruments and different brands is also a direction worth exploring.This study only focused on whether the digestion solution was complete from a qualitative analysis perspective.In the future, it could be considered from a perspective of quantitative analysis, for example, selecting a tire with a known heavy metals content and using different methods to dissolve it to see which is closer to the standard.

Figure 1 .
Figure 1.The contamination of tires.

Figure 2 .
Figure 2. The completion process for the determination of heavy metals in tires.

Figure 3 .
Figure 3.The effect of digestion with different combinations of acids.

Figure 5 .
Figure 5.The effect of digestion with different maximum temperatures.

Figure 6 .
Figure 6.The effect of digestion with different holding times.

Figure 8 .
Figure 8.The effect of digestion with different grams of sample.

Table 2 .
Maximum temperature variation of step 2.

Table 4 .
The effect of digestion with different holding times.

Table 6 .
The digestion effect of different grams of sample.

Table 7 .
Microwave digestion schemes for some complex materials.

Table 8 .
Microwave digestion schemes for rubber tires.